The present invention relates to a forming method for a ferroelectric thin film which can reduce oxygen defects in a dielectric, and also to a manufacturing method for a nonvolatile memory using the forming method for the ferroelectric thin film.
With the recent progress of film deposition techniques, research of applications of a nonvolatile memory cell using a ferroelectric thin film has increasingly been developed. This nonvolatile memory cell is a high-speed rewritable nonvolatile memory cell utilizing the high-speed polarization inversion and the residual polarization of the ferroelectric thin film. The nonvolatile memory cell using the ferroelectric thin film now being researched may be classified into a type of detecting a change in quantity of stored charge in a ferroelectric capacitor and a type of detecting a change in resistance due to spontaneous polarization of a ferroelectric. The nonvolatile memory cell in the present invention belongs to the former type.
An example of the nonvolatile memory cell of the type of detecting a change in quantity of stored charge in a ferroelectric capacitor is a nonvolatile memory cell having a 1-capacitor+1-transistor structure configured by adding a select transistor to a ferroelectric capacitor. The ferroelectric capacitor is composed of a lower electrode, an upper electrode, and a ferroelectric thin film sandwiched between these electrodes.
In regard to materials of the ferroelectric thin film, a related art PZT ferroelectric shows a so-called fatigue phenomenon, i.e., a reduction in residual polarization with repetitions of rewriting of data. However, this fatigue phenomenon is not exhibited in a Bi-based layer-structural perovskite type ferroelectric thin film (which will be referred to also as a bismuth layered ferroelectric thin film), so that this material has now received attention as the material for the nonvolatile memory. In applying the bismuth layered ferroelectric thin film to the nonvolatile memory, formation of this thin film is an especially important technique, and development has now been pursued on a spin-coat process such as a MOD process (Metal Organic Decomposition) process and a CVD (Chemical Vapor Deposition) process.
However, a ferroelectric thin film obtained by any of these processes lacks oxygen, and tends to be formed as a film containing oxygen defects (oxygen vacancies). The oxygen defects (oxygen vacancies) cause a reduction in permittivity, an increase in leakage current, a reduction in residual polarization, an increase in coercive electric field, etc., thus making a large trouble in obtaining desired dielectric characteristics and ferroelectric characteristics.
The occurrence of oxygen defects (oxygen vacancies) during formation of the ferroelectric thin film by the MOD process or the CVD process is considered to be due to the following. In the case of film deposition by the MOD process, a thin film is first deposited from source compounds having no metal-oxide bonds, and is next subjected to heat treatment in an oxygen atmosphere. As a result, metal-oxygen bonds based on solid-phase diffusion of oxygen are formed by this heat treatment to thereby form a ferroelectric thin film as an oxide. Accordingly, oxygen defects (oxygen vacancies) tend to be generated in the ferroelectric thin film formed by the MOD process. On the other hand, also in the case of film deposition by the CVD process, the oxidized state of a thin film to be deposited greatly depends on whether or not source compounds have metal-oxygen bonds. At present, commercially available source materials for bismuth (Bi) are triphenylbismuth and its analogous compounds only, and these compounds have no metal-oxygen bonds. Accordingly, in the case of depositing a bismuth layered ferroelectric thin film by the CVD process, the following two-step reaction is required.
(A) Decomposition of triphenylbismuth and production of metallic bismuth. PA1 (B) Oxidation of the metallic bismuth by oxygen in the oxygen atmosphere.
Therefore, it is hard to avoid formation of a ferroelectric thin film containing oxygen defects (oxygen vacancies). Accordingly, even by both the processes, that is, the MOD process and the CVD process, solution of the problem of oxygen defects (oxygen vacancies) would be very difficult in the absence of development of new source compounds or development of a new forming method for a ferroelectric thin film.
In these circumstances, the best forming method for a ferroelectric thin film having desired crystallinity and physical properties to solve the problem of oxygen defects (oxygen vacancies) is considered to have a step of depositing a ferroelectric thin film and a step of postannealing the ferroelectric thin film. As a postanneal process for a thin film of oxide materials, heat treatment (anneal) in an oxygen gas is generally known. However, in many cases, this process cannot sufficiently remove oxygen defects (oxygen vacancies) from the ferroelectric thin film. As a result, unoxidized bismuth remains as a metal in the surface layer of the ferroelectric thin film. Further, since the heat treatment (anneal) in the oxygen gas is performed usually at about 800.degree. C., there occurs another problem such that defects tend to be generated in the ferroelectric thin film by the heat in the heat treatment.